1. Field of the Invention
The present invention relates to an image projection apparatus that displays an image and then projects it, and more particularly to an image projection apparatus employing a reflective display device that displays an image through angle modulation of pixels.
2. Description of Related Art
Many image projection apparatuses use a reflective display device that displays an image through angle modulation of pixels. Such an image projection apparatus is provided with an illumination optical system for directing illumination light to the display device and a projection optical system for projecting image light (that is, light representing an image) from the display device.
Here, the illumination optical system should not intercept the image light (hereinafter also the “projection light”), nor should the projection optical system intercept the illumination light. Thus, the illumination optical system and the projection optical system need to be disposed on separate optical paths, one for the illumination light and another for the projection light. In addition, for miniaturization of the image projection apparatus as a whole, it is preferable that the illumination light and the projection light be separated within a narrow space.
To cope with these requirements, for example, in one conventionally proposed optical projection system (image projection apparatus) disclosed in Japanese Laid-Open Patent Application No. H8-254662 (hereinafter Patent Publication 1), a field lens is provided close to the display device so that improved separation between projection light and illumination light is achieved by the light-converging action of the field lens.
In another conventionally proposed image projection apparatus, a display device is used in which the deflection axis of the pixels runs at 45 degrees relative to both the longer and shorter sides of the display area, and a TIR prism having an air gap is disposed immediately in front of the display device. In this image projection apparatus, through total reflection of selectively either illumination light or projection light, separation between the illumination light and the projection light is achieved.
Image projection apparatuses provided with a field lens, however, are prone to suffer from reflection at the interfaces of the field lens. Such reflection causes ghosts and diminishes contrast. Thus, with these image projection apparatuses, it is impossible to obtain satisfactory image quality.
On the other hand, image projection apparatuses provided with a TIR prism combined with a reflective display device in which the deflection axis of the pixels runs at 45 degrees relative to every side of the display area are already commercially available. These image projection apparatuses, however, are expensive just because the TIR prism itself is expensive. In addition, these image projection apparatuses suffer from a significant loss of light at the air gap surfaces of the TIR prism. Thus, to present images with satisfactory brightness, it is essential to use a high-output light source.